This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
There is a need for high throughput assembly of small objects (semiconductor chips, LED chips, etc.) into a large area device (display, heterogeneous optoelectronic systems, biomedical devices, etc.). Incompatibility of substrate materials (for example, plastics) to fabrication technology of devices (for example, LED chips) requires the process of assembly some objects on a common device substrate of different origin.
Bible [U.S. Pat. No. 8,333,860] suggested an array of electrostatic MEMS heads for selective pick up and transfer objects on a receiving substrate. This technology required manufacturing transfer devices using MEMS technology. Transfer large amounts of microdevices (chips) onto a single device (display) requires higher than 99.99% yield of transfer, which is very problematic to achieve with a large array of MEMS electrostatic actuators.
Nuzzo [U.S. Pat. No. 7,943,491] suggested a method and system for such assembly based on elastomeric transfer device, which uses kinetically control adhesion to elastomeric surface to transfer objects from one substrate to another. In this patent application high separation rates are used to generate adhesion forces large enough to transfer objects from a donor substrate to elastomeric transfer device, and low separation rates have to be used in order to generate adhesion forces low enough to facilitate transfer of objects from transfer device to a receiver substrate. The difference in those rates could be as high as 100 times (100 mm/s rates for pickup objects from donor and 1 mm/s assembly rate to receiver). This inevitably puts an inherent limit on throughput of such system. Moreover, relying on just surface energy for proper transfer of a very large quantities of materials is problematic. Elastomeric material (for example, polydimethylsiloxane) is quite porous and can accumulate moisture, solvents and other vapors easily from the ambience or from an object in contact, which could change surface energy significantly. Also, exposure of elastomer (PDMS) to light (especially UV component) could change surface energy too. The content of PDMS is not usually uniform, having different molecular weight components moving to and from the surface over time and at different temperatures, which could change surface energy in a very unpredictable way. All above would inevitably impact the robustness of the process of pick-up and separation of objects and elastomeric transfer device. At the same time, the assembly process must delivery higher than 99% yield to be adopted in high volume low cost manufacturing process.
Here we suggest a new method for objects transfer and assembly, which does not rely only on surface energy of elastomeric material, but uses ultrasonic energy, and in some embodiments UV light, to facilitate separation of objects from elastomeric material and bonding them to a receiver substrate.